Detection and Suppression of Electrical Arcing

ABSTRACT

Method and apparatus for detecting or suppressing electrical arcing or other abnormal change in the electrical impedance of a load connected to a power source. Preferably the load is a plasma chamber used for manufacturing electronic components such as semiconductors and flat panel displays. Arcing is detected by monitoring one or more sensors. Each sensor either responds to a characteristic of the electrical power being supplied by an electrical power source to the plasma or is coupled to the plasma chamber so as to respond to an electromagnetic condition within the chamber. Arcing is suppressed by reducing the power output for a brief period. Then the power source increases its power output, preferably to its original value. If the arcing resumes, the power source repeats the steps of reducing and then restoring the power output.

CROSS REFERENCE TO RELATED APPLICATION

This patent application is a continuation of application Ser. No.11/149,896 filed Jun. 10, 2005, now U.S. Pat. No. 7,292,045 issued Nov.6, 2007.

FIELD OF THE INVENTION

The invention relates generally to apparatus and methods for detectingor suppressing electrical arcing or any other abnormal change in theelectrical impedance of a load to which a power source is supplyingelectrical power. Preferably the load is a plasma chamber used formanufacturing electronic components such as semiconductors and flatpanel displays, and the abnormal change that the invention detects orsuppresses comprises electrical arcing within the plasma chamber.

BACKGROUND OF THE INVENTION

Plasma chambers commonly are used for performing various processes suchas chemical vapor deposition, sputter deposition and plasma-enhancedetching processes used in manufacturing an electronic workpiece such asa semiconductor device or flat panel display. Commonly the plasma withina plasma chamber is sustained by coupling RF or DC power from anelectrical power source to the plasma. The coupling is accomplishedtypically by connecting the power source to an electrode within thechamber or to an antenna or magnetic coil within or adjacent to thechamber.

The conditions within a plasma chamber generally change during theprogression of the manufacturing process being performed within thechamber, and such changes sometimes cause electrical arcing within thechamber. If any electrical arcing occurs between the plasma and theworkpiece being manufactured, or between the plasma any of the chambercomponents, damage may occur to the workpiece or the chamber components.The risk of damage increases with the duration of the arcing.

To minimize or prevent such damage, there is a need for apparatus thatcan quickly detect electrical arcing within the plasma chamber or anyother abnormality that changes the electrical load impedance that thechamber presents to the power source. In addition, a need exists forapparatus that can suppress (i.e., stop) the arcing while allowing theplasma process to continue with as little interruption as possible.

More generally, there is a need for apparatus that can quickly detect orsuppress abnormal changes in an electrical load to which a power sourceis supplying electrical power.

SUMMARY OF THE INVENTION

One aspect of the invention is an apparatus and method of detectingelectrical arcing in a plasma chamber by monitoring one or more sensors.Each sensor either responds to a characteristic of the electrical powerbeing supplied by an electrical power source to the plasma or is coupledto the plasma chamber so as to respond to an electromagnetic conditionwithin the chamber. The electromagnetic condition detected by a sensorcoupled to the plasma chamber can be, for example, an electric field, amagnetic field, or optical radiation within the chamber.

The monitoring preferably comprises detecting when an electrical signalproduced by a sensor, or a value derived from (i.e., as a function of)one or more such sensor signals, crosses a threshold. An example of avalue derived from a sensor signal is the rate of change of the sensorsignal.

When the power source is an RF power source, a sensor responsive to theRF power supplied to the plasma chamber preferably comprises an RFdirectional coupler connected between the RF power source and the plasmachamber that detects one or more of forward RF power, reflected RFpower, and reflected power coefficient. The monitoring preferablycomprises detecting the occurrence of arcing when the reflected RF poweror the reflected power coefficient exceeds a first threshold or the rateof change of the reflected power or reflected power coefficient exceedsa second threshold.

A second aspect of the invention is adapted to manufacturing systems inwhich a manufacturing process controller controls the output powerproduced by the electrical power source so that it produces differentlevels of power at different steps or stages of a manufacturing process.In this aspect of the invention, the aforesaid one or more thresholdsare adjusted dynamically in response to the power level currentlyselected by the process controller. An advantage of this invention isthat it enables arcing to be detected with high sensitivity because thethresholds can be closely tailored to the power output of the powersource.

A third aspect of the invention is an apparatus and method ofsuppressing arcing without stopping the plasma process that is beingperformed in the plasma chamber. When arcing is detected, the powersource briefly reduces its output power, preferably to zero power.Normally this will eliminate the arcing. After a very brief period oftime, typically a few milliseconds or less, the power source increasesits power output, preferably to its original value. If the arcingresumes, the power source reduces its power again for a brief period.Typically the arcing will completely disappear after a few repetitionsof these brief periods of reduced power, so that the plasma processbeing performed in the chamber typically can resume after aninterruption of only ten milliseconds or so.

A fourth aspect of the invention is an apparatus and method of detectingan abnormal change in the electrical impedance of an electrical load towhich an electrical power source is connected. The fourth aspect is ageneralization of the first aspect to include abnormalities in impedanceother than arcing and to include an electrical load that need not be aplasma chamber. An abnormal change in the electrical impedance of theload is detected by monitoring one or more sensors that respond to acharacteristic of the power being supplied by the power source to theload. The monitoring preferably comprises detecting when an electricalsignal produced by a sensor, or a value derived from one or more sensorsignals, crosses a threshold. An example of a value derived from asensor signal is the rate of change of the sensor signal.

When the power source is an RF power source, a sensor preferablycomprises an RF directional coupler connected between the RF powersource and the load that detects one or more of forward RF power,reflected RF power, and reflected power coefficient. The monitoringpreferably comprises detecting an abnormal change when the reflected RFpower or the reflected power coefficient exceeds a first threshold orthe rate of change of the reflected power or reflected power coefficientexceeds a second threshold.

A fifth aspect of the invention is an apparatus and method of reversing(i.e., returning to normal) an abnormal change in the electricalimpedance of an electrical load to which an electrical power source isconnected. The fourth aspect is a generalization of the third aspect toinclude abnormalities in impedance other than arcing and to include anelectrical load that need not be a plasma chamber.

When an abnormal change in the electrical impedance of the load isdetected, the power source briefly reduces its output power, preferablyto zero power. This may eliminate the abnormality. After a very briefperiod of time, typically a few milliseconds or less, the power sourceincreases its power output, preferably to its original value. If theabnormality is detected again, the power source reduces its power againfor a brief period. The abnormality may completely disappear after a fewrepetitions of these brief periods of reduced power, so that the normaloperation of the power source can resume.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of the arc detection and suppressioncircuit in combination with an RF power source and plasma chamber.

FIG. 2 is a graph that simulates electrical signals during arcing withinthe plasma chamber.

FIG. 3 is a flow chart of the basic arc detection algorithm.

FIG. 4 is a flow chart of an improved arc detection algorithm thatrejects identifications of arcing shorter than a minimum durationthreshold.

FIG. 5 is a flow chart of a further modified arc detection algorithmthat adds hysteresis for the arc detection thresholds and that updatesthe thresholds at every sampling interval as a function of the power setpoint of the electrical power supply.

FIG. 6 is a flow chart of the algorithm of FIG. 5 modified to comparethe arc detection thresholds with values derived from the sensorsignals.

FIG. 7 is a schematic block diagram of a threshold detector implementedwith analog voltage comparators.

FIG. 8 is an electrical schematic of a threshold adjustment circuithaving a potentiometer.

FIG. 9 is a flow chart of an arc suppression algorithm.

FIG. 10 is an alternative embodiment of the circuit of FIG. 1 includinga multiplexor connected to the RF set point input of the RF powersource.

FIG. 11 is another alternative embodiment of the circuit of FIG. 1 inwhich the DSP provides a power set point signal to the RF power source.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

1. Hardware

FIG. 1 shows an apparatus that can both detect arcing and suppressarcing according to different aspects of the present invention. Theapparatus includes an electrical circuit 10 referred to herein as the“arc detection and suppression circuit” or “ADSC”. The ADSC 10implements our novel arc detection and suppression algorithms asdescribed below. The ADSC can be a digital circuit such as aprogrammable computer, an analog electrical circuit, or a combination ofdigital and analog circuits, examples of which are described below.

As stated in the above Summary of the Invention, our arc detection andsuppression circuit also is useful more generally to detect or suppressabnormalities in impedance other than arcing. In this more general case,the ADSC 10 can be referred to as the “impedance abnormality detectionand suppression circuit”. Suppression of an impedance abnormality meansreversing the abnormal condition so as to return the impedance to itsnormal value or normal range of values. Although the followingdescription of the preferred embodiments repeatedly refers to arcing,the term “arcing” can be replaced by “impedance abnormality” throughoutthe description.

Similarly, as stated in the Summary of the Invention, the invention alsois useful for an electrical load that need not be a plasma chamber. Anabnormal change in the electrical impedance of the load is detected bymonitoring one or more sensors that respond to a characteristic of thepower being supplied by the electrical power source to the load.Although the following description of the preferred embodimentsrepeatedly refers to a plasma chamber, the plasma chamber can bereplaced by any electrical load.

The illustrative manufacturing system to which the ADSC 10 is connectedincludes an RF power source 20 having an RF output 22 connected to theinput of an RF directional coupler 24. (Commercial RF power generatorsoften combine an RF power source and directional coupler within a singleenclosure.) The directional coupler has an RF output 26 connected to theinput of an RF impedance matching network 28. The directional couplersenses or measures the forward and reflected RF power propagating fromthe RF power source 20 to the matching network 28 and produces forwardand reflected sensor output signals 25, 27 whose values respectivelyrepresent the forward and reflected RF power measurements S₁ and S₂. Theoutput of the RF matching network is connected to an electrode 32 withina plasma chamber 30. The components described in this paragraph areincluded in conventional manufacturing systems with RF-powered plasmachambers.

The invention also is applicable to electrical power sources that supplypower other than RF power, such as a DC power source connected to supplypower to a plasma sputtering chamber. When the electrical power sourceis a DC power source, there will be no reflected power, but all othersensor signals described below, including forward electrical power, canbe used to detect and suppress electrical arcing or other abnormalimpedance changes in the electrical load to which the power source isconnected to supply power. Accordingly, all references to an RF powersource, its enable input, and its power set point input apply equally toa DC power source.

Each manufacturing process performed within the plasma chamber 30conventionally is controlled by a process controller 34, which is acomputer that stores and executes a process sequence program so as todirect and control the actions and parameters of the manufacturingprocess. For example, the process controller 34 conventionally isprogrammed to control robots that move workpieces in and out of theplasma chamber, to control gas valves that regulate the flow of variousreagent gases into the chamber, and to control an exhaust valve thatregulates the gas pressure within the process chamber. The processcontroller conventionally is supplied as a component of either theprocess chamber 30 or a multi-chamber platform or mainframe to which theprocess chamber is connected.

Many manufacturing processes include a sequence of process steps thatare performed at different RF power levels. Accordingly, an additionalconventional function of the process controller 34 is to control the RFpower output level of the RF power source 20 so that the latter suppliesdifferent levels of RF power to the plasma chamber 30 at different stepsor stages of the process. Specifically, the process controller 34 sendsto the RF power control input 21 of the RF power source 20 an RF powerset point signal 36 (FIG. 2A) which is a digital or analog electricalsignal whose value at any point in time represents the level of RF powercurrently specified by the process sequence being executed by theprocess controller.

The RF power source 20 responds to the RF power set point signal 36 byattempting to adjust the actual power level of the RF signal produced atits RF output 22 so that the actual RF power level equals the desired RFpower level represented by the value of the RF power set point signal36. As explained below, an abnormal condition in the process chamber,such as an electrical arc, sometimes can prevent the RF power source 20from producing the RF power level designated by the RF power set pointsignal 36.

In preferred embodiments of the present invention, the RF power setpoint signal 36 is additionally connected to the ADSC 10 so that theADSC can detect when transitions in RF power level are specified by theprocess sequence currently being executed by the process controller.

The steps of the arc detection and arc suppression algorithms describedin the remainder of this patent specification are performed by the ADSC10. The ADSC can consist of a single conventional computer processorthat is programmed to perform all the functions described below. Whilesuch approach is expected to be economically feasible in the nearfuture, presently a single processor fast enough to perform all suchfunctions would be undesirably expensive. Accordingly, our preferredembodiment of the ADSC includes two computer processors: a relativelyslow processor which we refer to as the programmable logic controller(PLC) 12, and a much faster processor which we refer to as the digitalsignal processor (DSP) 14. The ADSC also may include analog electricalcircuits such as analog-to-digital converters and analog slope detectorsor differentiators as described below.

Both the PLC and the DSP include a programmable arithmetic computationunit and a memory for storing input data, output data, and parametersused by the algorithms described below. The DSP samples a number ofsensor signals, optionally derives other values from the sensor signals,and compares them with thresholds (defined below) in real time.Preferably, the DSP is fast enough to perform all these steps in lessthan one microsecond. The PLC can be slower because it performsinterface and logging functions that are less time critical.

The preferred division of functions between the PLC 12 and DSP 14 isincluded in the following descriptions of the arc detection and arcsuppression algorithms. However, any function described below asperformed by the DSP or the PLC can be performed instead by any analogor digital circuit or computer processor within the ADSC, such as asingle computer processor that performs all the functions of the ADSC.

2. Detection of Arcing

The arc detection and suppression computer (ADSC) 10 is programmed toperform both the arcing suppression algorithm described below under theheading “Arcing Suppression” and the arcing detection algorithmdescribed in the following paragraphs.

The DSP detects the occurrence of an electrical arc within the plasmachamber 30 by monitoring in real time sensor output signals S_(i) (S₁,S₂, S₃, etc.) produced by one or more sensors and detecting when any ofthe sensor output signals S_(i) has an abnormal value suggestive ofarcing within the plasma chamber.

One broad category of sensor whose output can be used to detect arcingwithin the plasma chamber 30 is a sensor that is coupled to the plasmachamber so as to sense an electromagnetic condition within the chamber,such as an electric field, a magnetic field, or optical radiation withinthe chamber. An example of such a sensor is an optical sensor (notshown) that monitors light radiated by the plasma. Another example is anEMF probe or antenna 47 mounted within the plasma chamber. The antennais connected to the input of an electromagnetic field (EMF) detector 46,which produces a sensor output signal 48 whose value represents themeasurement of the electromagnetic field strength received by theantenna. The output 48 of the EMF detector is connected to the DSP 14.

Another broad category of sensor whose output can be used to detectarcing within the plasma chamber 30 is a sensor that is coupled betweenan RF power source 20 and the plasma chamber so as to sense (i.e.,detect) a characteristic of the RF power being supplied by the RF powersource to the plasma.

One example of such a sensor is a conventional directional coupler 24connected between the RF power source 20 and the RF impedance matchingnetwork 28, shown in FIG. 1. The illustrated directional couplerproduces two sensor output signals 25, 27 whose respective amplitudesrespectively represent the forward RF power and reflected RF powersensed by the directional coupler. Other directional couplers mayproduce sensor outputs that represent either the ratio between thereflected and forward RF power or else the square root of this ratio,known as the reflected power coefficient. As stated above, commercial RFpower generators often combine an RF power source and directionalcoupler within a single enclosure. If the forward and reflected poweroutputs 25, 27 of the directional coupler are RF rather than DC signals,a conventional envelope detector (i.e., a rectifier followed by a lowpass filter) should be connected between each output of the directionalcoupler and the ADSC.

A second example of a sensor that senses a characteristic of the RFpower is an RF distortion analyzer, such as a harmonic sensor, thatdetects distortion of the RF voltage or current supplied by the RF powersource to the chamber by measuring the amplitude of any spectralcomponents of said voltage or current at frequencies substantiallyhigher than the fundamental frequency of the RF voltage or current.

A third example of such a sensor is an electrically tunable RF matchingnetwork 28 having inductors or capacitors whose values are adjusted inresponse to electrical control signals in order to tune the matchingnetwork. These electrical control signals are useful as sensor outputsignals because changes in the control signals signify changes in theelectrical impedance presented by the plasma chamber to the output ofthe RF matching network. Similarly, other types of RF matching networks28 are tunable by adjusting the frequency of the RF source 20. With suchfrequency-tunable matching networks, the frequency of the RF source orthe output of a controller that controls such frequency can constitute asensor output signal S_(i).

FIG. 2 graphically illustrates how the forward RF power signal 25 andreflected RF power signal 27 produced by a directional coupler 24 can beused to detect the occurrence of arcing within the plasma chamber 30.FIG. 2A shows the RF power set point signal 36 that the processcontroller 34 sends to the RF power source 20. Its value represents whatthe amplitude of the RF power output 22 should be if both the RF powersource and the load impedance presented by the plasma chamber 30 werefree of fluctuations and instabilities. FIG. 2 illustrates amanufacturing process being performed in the plasma chamber thatrequires a first level of RF power P₁ between times t₀ and t₃ and alower level of RF power P₂ between times t₃ and t₄.

FIGS. 2B and 2C respectively show the forward RF power signal 25 andreflected RF power signal 27 detected by the directional coupler 24,which in this example are the first and second sensor signals S₁ and S₂.Arcing within the plasma chamber begins at time t₁ and ends at time t₂.During the interval t₁-t₂ in which arcing occurs, the forward RF powerdrops and the reflected RF power increases. In our invention, we detectthis change in forward and reflected RF power to identify arcing withinthe plasma chamber.

An optical sensor or EMF sensor coupled to the interior of the plasmachamber, such as RF antenna 47, would behave similarly to reflectedpower signal 27 graphed in FIG. 2C. Specifically, any of these sensorswould have a relatively low steady state value with some noise orinstability superimposed thereon, and would have a substantial increasein value when arcing occurs within the plasma chamber.

We have found the reflected RF power signal 27 the single most reliableindicator of arcing because its value is relatively stable in theabsence of arcing. The forward RF power signal 25, the EMF sensor signal48, and the slope (time derivative) of the reflected RF power signalalso are useful signals to supplement the reflected RF power signal, butthey are characterized by more instability or noise.

In the present invention, each sensor signal S_(i) is considered toindicate that arcing is occurring according to whether its output signalis below or above a threshold L_(i), referred to herein as an arcingdetection threshold. Some sensor signals, such as the forward RF power25 (S₁), indicate the occurrence of arcing when their value drops belowa threshold, while other sensor signals, such as the reflected RF power27 (S₂) and EMF detector 46-48 (S₃), indicate the occurrence of arcingwhen their value exceeds a threshold. To adopt a consistent nomenclaturethat covers both types of sensor signals, we will refer to the resultR_(i) of a comparison between a sensor output signal S_(i) and itscorresponding arcing detection threshold L_(i) as being positive ornegative according to whether the comparison does or does not indicatethe occurrence of arcing, and we will refer to a sensor output signalhas having a bad value or a good value according to whether the resultR_(i) of comparing the sensor output signal S_(i) to its correspondingthreshold L_(i) is positive or negative. Accordingly, the forward RFpower sensor signal 25 (S₁) is considered bad if it drops below a firstthreshold L₁, whereas the reflected RF power sensor signal 27 (S₂) andthe EMF detector signal 48 (S₃) are considered bad if they exceed asecond threshold L₂ and a third threshold L₃, respectively.

FIG. 3 shows the algorithm performed by the ADSC 10 of FIG. 1. First,the ADSC (either the PLC 12 or the DSP 14, as explained below)establishes the values of the first, second and third arcing detectionthresholds L₁, L₂ and L₃ and stores the threshold values in a memorywithin the DSP (Step 100). Alternative methods of establishing thearcing detection thresholds are described below under the heading“Adjusting Arc Detection Thresholds”.

In real time, the DSP periodically samples (Step 104) the values of eachof the sensor signals S₁ (forward RF power signal 25), S₂ (reflected RFpower signal 27) and S₃ (EMF detector signal 48). The term “sample”means to receive the specified electrical signal and store itsinstantaneous value at a selected point in time, as can be done by aconventional sample-and-hold circuit within the DSP.

The DSP compares the current sampled values of the first, second andthird sensor signals S₁, S₂ and S₃ to the first, second and thirdthresholds L₁, L₂ and L₃, respectively (Step 108). The DSP signals analert that an arc has occurred (Step 150) if the result of any of thesecomparisons is positive, that is, if the first sensor signal S₁ (forwardRF power signal 25) drops below the first threshold L₁, if the secondsensor signal S₂ (reflected RF power signal 27) exceeds the secondthreshold L₂, or if the third sensor signal S₃ (EMF detector signal 48)exceeds the third threshold L₃.

Preferably, the DSP signals that an arc has occurred (Step 150) bysending an arc alert signal over electrical communications bus 15 to thePLC (FIG. 2D). The PLC can be programmed to specify what remedial actionit should perform when it receives an arc alert signal from the DSP(Step 152). Examples of appropriate remedial actions are: (1) to performthe arc suppression algorithm described below in Section 9 entitled“Suppression of Arcing”, i.e., Steps 200-206 in FIG. 9; (2) to send acommand, via communications bus 16, directing the process controller 34to shut down the RF power to the plasma chamber immediately; (3) to senda command, via communications bus 16, directing the process controller34 to wait until completion of processing of the workpiece currentlywithin the plasma chamber, and then shut down the operation of theplasma chamber; or (4) to signal an alarm, via external communicationsbus 18, to the person operating the chamber and/or log the event incomputer memory, but not disrupt operation of the plasma chamber.

After the results R_(i) of all comparisons become negative—i.e., afterall sensor signals S_(i) return to the “good” side of their respectivethresholds L_(i)—the DSP clears or removes the arc alert signal 15 asshown in FIG. 2D at time t₂ (Step 110).

3. Sampling Interval and Minimum Arc Duration

The steps of sampling the sensor signals (Step 104) and comparing eachsensor signal to its corresponding arc detection threshold (Step 108)are periodically repeated at an interval referred to as the samplinginterval ΔT.

If arcing occurs with a duration less than the sampling interval, thesensor signals S_(i) can change from good to bad and then return to goodduring the interval between successive samples of the sensor signals(Step 104), in which case the ADSC would not detect that arcing hadoccurred. Such failure to detect arcing is not necessarily undesirableif the duration of the arcing is too short to create any risk ofdamaging either the workpiece being manufactured in the plasma chamberor any components of the chamber. Conversely, it is important for theADSC to reliably detect any arcing of a duration great enough to risksuch damage. Therefore, the sampling interval preferably should be nogreater than the shortest arc duration that creates a substantial riskof such damage. For the reasons explained below, the sampling intervalmore preferably is no more than one-fourth or one-tenth of suchduration.

Random noise or instability in a sensor signal can cause that sensorsignal to briefly cross its respective threshold even though arcing isnot occurring. To prevent such noise spikes from being misinterpreted asan occurrence of arcing, the arcing detection algorithm preferablyrequires a positive comparison result (i.e., a sensor having “bad”values) to persist for at least a minimum time, referred to herein asthe minimum arc duration threshold “D”, before the arc detectionalgorithm signals that arcing has occurred. To ensure the detection ofpotentially damaging arcing events, the minimum arc duration thresholdpreferably should be no greater than the shortest arc duration thatcreates a substantial risk of damaging either the workpiece beingmanufactured in the plasma chamber or any components of the chamber.

When the step of comparing each sensor signal to its correspondingthreshold is performed by a digital circuit at a periodic samplinginterval, as in all embodiments described herein other than the analogcomparison circuit of FIG. 7, erroneous arc detection in response tosensor signal noise or instability can be avoided by requiring apositive comparison result (Step 106) to persist for at least aplurality “N” of consecutive sampling intervals. This plurality N,referred to herein as the positive comparison count threshold,preferably is at least three. In our current preferred embodiment, N=10.

The minimum arc duration threshold D will be the product of the samplinginterval ΔT and the positive comparison count threshold N. In designingan implementation of the arc detection system, the designer shouldchoose the minimum arc duration threshold and the positive comparisoncount threshold based on the considerations set forth above. Thesampling interval ΔT should be less than or equal to the chosen minimumarc duration threshold D divided by the chosen positive comparison countthreshold N.

FIG. 4 illustrates an arc detection algorithm that includes Steps 112,120 and 130 for related to counting the number of positive comparisonresults and signaling an alert that arcing has occurred only if thecount is greater than or equal to the positive comparison countthreshold N. The other steps in the algorithm are the same as thepreviously discussed algorithm of FIG. 3.

Steps 112, 120 and 130 preferably are performed by the DSP. The DSPstores within its memory the positive comparison count threshold N. TheDSP also stores within its memory a numerical count whose value iscleared, i.e., reset to zero, whenever all the results R_(i) of all thecomparisons between the sensor signals S_(i) and their corresponding arcdetection thresholds L_(i) are negative (Step 112). Conversely, in anysampling interval in which at least one such comparison has a positiveresult, the DSP increments the count stored in its memory (Step 120).Each time it is incremented, the count is compared with the positivecomparison count threshold N (Step 130). If the count is equal to orgreater than N, the DSP signals an alert via communications bus 15 asdescribed above (Step 150).

The invention was tested with a plasma chamber 30 used for depositingfilms on a glass substrate used for fabricating flat panel displayshaving arrays of thin film transistors. These tests suggest that 10microseconds is a reasonable threshold between arcs that are too shortto create damage and arcs that pose a substantial risk of damaging sometransistors on the substrate. Therefore, to minimize the risk of falsealerts, Step 130 requires a sensor's abnormal output indicatingarcing—i.e., one or more positive comparison results in Step 108—topersist for a minimum arc duration threshold D of at least 10microseconds before signaling an alert that arcing has occurred (Step150). In the preferred embodiment, the positive comparison countthreshold N is 10, and the sampling interval ΔT is 1 microsecond.

If the objective of detecting arcs is primarily to detect arcs ofduration great enough to crack or break the workpiece rather than todetect short duration arcs that might damage transistors on theworkpiece, the minimum arc duration threshold could be as long as 100 msor even 1 second, since we have observed that arcs as long as 100 mstypically do not damage a workpiece that is a glass substrate.

The ADSC optionally can store in its memory a log that records the valueof the sensor signals every time a comparison has a positive result(Step 122), irrespective of whether the number of positive comparisonresults is sufficient to signal an arcing alert. Such log may be usefulfor later analysis to improve the performance of the manufacturingprocess. There is no need for the log to be updated in real time, so theDSP 14 can send the sensor values to the PLC 12 via the communicationsbus 15, and then the PLC can store the data in its own memory.

4. Adjusting Arc Detection Thresholds

The value of each arc detection threshold L_(i) should be stored in amemory within the DSP 14. These values can be established as part of theprogramming of the DSP 14. More preferably, a person operating theplasma chamber can enter the desired values of the arc detectionthresholds L_(i) into a factory control computer that transmits thethreshold values via communications bus 18 to the PLC 12, which thentransmits the threshold values L_(i) to the DSP via bus 15.

Each arc detection threshold L_(i) should be set to a value between (1)the range of values that the corresponding sensor signal S_(i) normallywould have in the absence of arcing, and (2) the range of values thatthe corresponding sensor signal S_(i) typically would have when arcingoccurs in the plasma chamber. As illustrated in FIGS. 2B and 2C, thesensor signals S_(i) in the absence of arcing fluctuate over a rangebecause of noise and instability in the RF power source 20 and in theload impedance of the plasma within the chamber, the latter being causedin part by changes in the plasma chemistry during the progress of themanufacturing process being performed in the plasma chamber.

More specifically, for each sensor measurement whose value increaseswhen an arc occurs, such as reflected RF power S₂ and chamber EMFdetector signal S₃, the corresponding arc detection threshold should behigh enough to avoid false detection of an arc when no arc actuallyoccurs, but low enough to avoid failure to detect an arc, i.e., lowenough so that any arc is reliably detected. In other words, each arcdetection threshold L_(i) should be low enough so that the correspondingsensor measurement S_(i) reliably exceeds the corresponding thresholdL_(i) when an arc occurs, and should be high enough so that thecorresponding sensor measurement S_(i) almost never exceeds thethreshold L_(i) in the absence of an arc.

The same principles apply to a sensor measurement whose value decreaseswhen an arc occurs, such as the forward power measurement S₁, exceptthat “low” and “below” are interchanged with “high” and “above”. Thatis, the arc detection threshold L₁ should be high enough so that theforward power S₁ reliably drops below the threshold L₁ when an arcoccurs, and should be low enough so that the forward power S₁ almostnever drops below the threshold L₁ in the absence of an arc.

The arc detection thresholds L_(i) can remain fixed throughout themanufacturing process as described above. However, the optimum values ofthe arc detection thresholds L_(i) may be different at different stepsof the process. Manufacturing processes performed within a plasmachamber often involve a succession of steps in which the RF power leveland/or gas chemistry within the chamber change from one step to thenext. For example, a process for forming an electronic device on aworkpiece may include a plasma-enhanced cleaning step, followed byplasma-enhanced chemical vapor deposition of one material, followed byplasma-enhanced chemical vapor deposition of a different material.

FIGS. 2A-2C illustrate a process in which a first process step isperformed during a first time interval t₀-t₃ at a first RF power levelP₁ and a second process step is performed during a second time intervalt₃-t₄ at a first RF power level P₂. FIG. 2B shows that the arc detectionthreshold L₁ with which the forward RF power signal S₁ is comparedduring the first time interval t₀-t₃ would be useless during theinterval t₃-t₄ because it is always greater than the forward RF powersignal S₁, and hence would produce a positive comparison result R_(i)throughout interval t₃-t₄, thereby erroneously indicating the occurrenceof arcing. To avoid this erroneous outcome, a lower arc detectionthreshold L_(1A) should be substituted for L₁ during the second timeinterval t₃-t₄.

One contemplated embodiment of the invention provides a different set ofarc detection thresholds L_(i) at different steps of the manufacturingprocess by storing within the memory of the PLC a table that specifiesfor each process step a start time, a stop time, and what the arcdetection sensor thresholds L_(i) should be for each of the sensorsbeing monitored by the DSP. The PLC would transfer to the DSP a new setof sensor thresholds L_(i) (L₁, L₂, L₃, etc.) at the beginning of eachnew process step. An advantage of this embodiment is that the thresholdvalues stored in the table can take into account differences in gaschemistry or other operating conditions during each process step, ratherthan merely basing the threshold values on the RF power level.

However, we have found that basing the arc detection threshold valuessolely on the value of the RF power set point signal 36 suffices forreliable arc detection. This method has the advantage of not requiringcustomizing the parameters stored in the memory of the PLC each time itis desired to perform a different manufacturing process in the plasmachamber. Accordingly, in our preferred embodiment the DSP is connectedto receive from the chamber process controller 34 the RF power set pointsignal 36 that the controller sends to the RF power source 20 to controlits output power. The RF power set point signal 36 instantaneouslyindicates to the DSP the intended RF output power at the current step ofthe process.

In this embodiment the DSP preferably repeats in every sampling intervalthe steps of receiving the current value of the RF power set pointsignal 36 and determining each arc detection threshold L_(i) as afunction of the RF power set point signal, as shown in FIG. 5 (Step100). The algorithm of this embodiment, shown in FIG. 5, differs fromthe algorithms of previously described embodiments (FIGS. 3 and 4) inthat Step 100 is inside the sequence of steps that is repeated for everysampling interval so that Step 100 is repeatedly performed shortlybefore the step of comparing each sensor measurement with itscorresponding threshold (Step 108). In the preferred embodiment, Steps100, 104 and 108 are repeated every microsecond.

In a simple but effective implementation, a multiplier constant K_(i) isassociated with each arc detection threshold L_(i). The thresholds areperiodically updated (Step 100) by setting the value of each arcdetection threshold L_(i) (L₁, L₂, L₃, etc.) to the product of itsrespective multiplier constant K_(i) (K₁, K₂, K₃, etc.) and the RF powerset point, i.e., the current value of the RF power set point signal 36.

To enable the DSP to rapidly update the arc detection thresholds L_(i)after any change in the RF power set point, the multiplier constantsK_(i) preferably should be stored in the memory of the DSP rather thanthe slower PLC, and the DSP preferably should perform the multiplicationof the thresholds L_(i) and the multiplier constants K_(i).

The values of the multiplier constants K_(i) can be determinedempirically. For example, where S₁ and S₂ are the forward and reflectedRF power, suitable values for K₁ and K₂ may be K₁=0.90 and K₂=0.03. Thismeans that an arc will be detected if the forward RF power S₁ dropsbelow 90% of the RF power set point or the reflected RF power S₂increases above 3% of the RF power set point.

Because multiplying requires much more computing power than adding,subtracting and comparing, avoiding multiplication steps in the arcdetection algorithm will enable the use of a less expensive DSP circuit.Consequently, our preferred embodiment does not use the multiplierconstants K_(i) described in the preceding paragraphs or any othermultiplying steps. Instead, the DSP determines the arc detectionthresholds L_(i) in response to the RF power set point signal 36 byperforming a table look-up. The look-up table is defined by dividinginto multiple ranges the various RF power set points that the processcontroller 34 is expected to specify during processing of a workpiece inthe plasma chamber, and then assigning to each power set point range avalue for each arc detection threshold L_(i). Each row in the tableincludes first and second columns for the minimum and maximum power setpoints that define the range to which a given table row applies, plus anadditional column for each arc detection threshold value L_(i).

The entire look-up table, i.e., the arc detection thresholds for eachrange of RF power set points, preferably is stored in the memory of theDSP. The DSP performs Step 100 of the algorithm shown in FIG. 5 byreading the current value of the RF power set point signal 36,determining which table row has minimum and maximum power set pointvalues that are respectively less than and greater than the current setpoint, and then setting each arc detection threshold L_(i) to the valuesspecified in the remaining columns of that table row.

For example, Table 1 illustrates a look-up table for an embodiment ofthe ADSC that uses only a single sensor signal, the reflected RF powersignal S₂. Table 2 illustrates a subset of the rows of a look-up tablefor an alternative embodiment of the ADSC that uses two sensor signals,the forward RF power signal S₁ and the reflected RF power signal S₂.TABLE 1 RF Power Set Point Range (watts) Minimum Maximum L₂ (watts) 02000 50 2001 4000 75 4001 6000 100 6001 8000 200 8001 10000 300

TABLE 2 RF power set point range (watts) Minimum Maximum L₁ (watts) L₂(watts) 1801 1900 1600 60 1901 2000 1700 60 2001 2100 1800 60 2101 22001900 60 2201 2300 2000 60 3901 4100 3500 120 4101 4300 3700 120

Besides showing Step 100 being inside the loop of steps that the DSPexecutes at every sampling interval, FIG. 5 further differs from FIGS. 3and 4 in that is shows Step 106 related to the feature of derivingvalues from the sensor signals, described below, and Steps 102, 114 and140 related to the hysteresis feature described below. Each of thesefeatures is an optional aspect of the present invention that can beincluded independently of whether the others are included, notwithstanding that they are included together in FIG. 5.

Although FIG. 5 shows the arc detection thresholds L_(i) being updatedat every sampling interval (Step 100), the arc detection thresholds canbe updated at a somewhat slower rate, in which case it may be possiblefor Step 100 to be performed by the slower PLC rather than the DSP. Inthis alternative embodiment, the PLC receives the RF power set pointsignal 36, establishes the values of the arc detection thresholds L_(i),and transmits the threshold values to the PLC via communications bus 15.The PLC can perform these steps asynchronously relative to the samplinginterval to which the DSP is synchronized in performing the steps ofsampling the sensor signals and comparing them with the arc detectionthresholds (Steps 104-140).

5. Hysteresis in Arc Detection Thresholds

An optional further improvement to the arc detection algorithm describedabove is to add hysteresis to the arc detection thresholds, by which wemean establishing different thresholds for identifying the start ofarcing and the end of arcing. FIG. 5 shows an embodiment of thealgorithm performed by the ADSC that includes the hysteresis feature.

When the DSP determines that arcing has begun and sends an arc alertsignal to the PLC or an external device (Step 150), the DSP enables ahysteresis mode (Step 140). The DSP preferably sets or clears a binarybit (a “flag”) in its memory to indicate whether the hysteresis mode isenabled.

While the hysteresis mode is enabled, the DSP will subtract from eacharc detection threshold L_(i) a corresponding hysteresis offset H_(i)(Step 102) each time the DSP compares the current sensor signals to thearc detection thresholds (Step 108). The subtraction can be a separatestep as shown in FIG. 5, or it can be part of the comparison operation(Step 108).

Each hysteresis offset H_(i) should be a positive number if itscorresponding threshold L_(i) is greater than the normal or “good”values of its corresponding sensor signal S_(i), and should be anegative number if its corresponding threshold L_(i) is less than thenormal or “good” values of its corresponding sensor signal S_(i).Therefore, the effect of the hysteresis offset is that the DSP requireseach sensor signal to return closer to its normal or ideal value beforethe DSP will recognize an arcing event as having ended (Step 110).

When the results R_(i) of all comparisons (Step 108) are negative andthe DSP removes (clears) the arc alert signal (Step 110), the DSP alsodisables the hysteresis mode (Step 114) so that the hysteresis offsetsno longer are subtracted from the arc detection thresholds (Step 102).

The hysteresis offsets can be included in the programming of the DSP orcan be transmitted by the person operating the chamber to the PLC viabus 18. More preferably, if the arc detection thresholds are stored in alook-up table as described above, the hysteresis offsets can be storedin the same table. We presently prefer a hysteresis value of 25 wattsfor both of the arc detection thresholds respectively corresponding tothe forward RF power and the reflected RF power sensor signals.

6. Disabling Arc Detection During Process Transitions

We have found that when the RF power is first turned on, or when the RFpower set point steps to a higher or lower level, the sensormeasurements S_(i) can temporarily change in a manner similar to theirchanges when an arc occurs. Specifically, when the RF power set pointincreases, the forward RF power S₁ can fail to increase at the same rateas the RF power set point 36. When the RF power set point eitherincreases or decreases, the resultant change in the plasma within thechamber can produce a temporary surge in both the reflected RF power S₂and the chamber EMF signal S₃. To avoid this behavior being erroneouslyidentified as an arc, a preferred embodiment of our invention disablesthe ADSC from performing the arc detection algorithm of FIGS. 3-5 duringthese periods of rapid change in the RF power set point.

Specifically, when the RF power is first turned on, the ADSC does notbegin performing the arc detection algorithm until after a predeterminedinitial suspension period which preferably exceeds the time required forthe RF power to increase to its final value and stabilize. Likewise, theADSC also suspends its performing of the arc detection algorithmwhenever the RF power set point signal 36 steps to a higher or lowervalue, as illustrated in FIG. 2A at times t₀ and t₃, or otherwiserapidly changes. Preferably the suspension period is at least 70 ms,more preferably at least 100 ms.

The DSP preferably is programmed to monitor the RF power set pointsignal 36 and detect when it changes substantially. In that event, theDSP should initiate a suspension of the arc detection algorithm for thepredetermined suspension period stored in the memory of the DSP. Duringthe suspension period, the DSP can update the arc detection thresholdsL_(i) in response to the new value of the RF set point (Step 100 of FIG.3 or 4). When the suspension period ends, the DSP should resumeperforming the arc detection algorithm at Step 104.

In this embodiment of the invention, because Steps 102-152 of the arcdetection algorithm are suspended whenever the RF power set pointchanges substantially, there is no need to perform the step of updatingthe arc detection thresholds, Step 100, except during each suspensionperiod. Therefore, it would not make sense to repeat Step 100 in everysampling interval as in the embodiment of the algorithm shown in FIG. 5.

7. Deriving Values from Sensor Signals

Besides directly monitoring the sensor signals S_(i) as described above,it can be useful to monitor one or more values V_(i) that we refer to as“sensor-derived values”, where each sensor-derived value is derived from(i.e., is a function of) one or more actual sensor signals S_(i). FIG. 6shows the arc detection algorithm of FIG. 5 modified to add the step ofderiving values from sensor signals (Step 106). In the comparison step(Step 108), each arc detection threshold L_(i) is compared with acorresponding sensor-derived value V_(i) instead of with a sensor signalS_(i) as in FIG. 5.

Examples of useful sensor-derived values are: (1) the sum or average ofmultiple sensor signals; (2) the ratio between two sensor signals, suchas the ratio between the reflected RF power 27 and the forward RF power25; and (3) the slope or time derivative of one of the sensor signals,such as the slope of the reflected RF power signal 27.

In addition, any of the sensor-derived values V_(i) can simply equal oneof the sensor signals S_(i), so that the step of comparing asensor-derived value with an arc detection threshold (FIG. 6, Step 108)is a generalization of, rather than an alternative to, the step ofcomparing a sensor signal to an arc detection threshold (FIGS. 3-5, Step108). Consequently, the FIG. 6 algorithm encompasses, and is ageneralization of, the FIG. 5 algorithm.

As in the arc detection algorithms discussed above, if a givensensor-derived value V_(i) normally is less than its corresponding arcdetection threshold L_(i) in the absence of arcing, than the resultR_(i) of the comparison in Step 108 is considered positive or negativeaccording to whether the sensor-derived value V_(i) is greater than orless than the threshold L_(i). Conversely, if a given sensor-derivedvalue V_(i) normally is greater than the threshold L_(i) in the absenceof arcing, then the result R_(i) of the comparison is consideredpositive or negative according to whether the sensor-derived value V_(i)is less than or greater than the threshold L_(i).

A combination of sensor-derived values we have found especiallyeffective for detecting arcing is to define a first sensor-derived valueV₁ as equal to the reflected RF power signal 27 and a secondsensor-derived value V₂ as equal to be the slope of the reflected RFpower signal. In the embodiment of FIG. 1, the DSP can be programmed tomathematically compute the slope of the reflected RF power signal basedon samples of the reflected RF power signal. Mathematical algorithms forcomputing the slope of a signal as a function of successive samples ofthat signal are well known. Alternatively, the embodiment of FIG. 7,described immediately below, employs an analog slope detector circuit toproduce a signal proportional to the slope of the reflected RF powersignal.

8. Distinguishing Long and Short Arcing Duration

Because the severity of damage caused by arcing increases with theduration of the arcing, it may be desirable for the ADSC to distinguishbetween short and long duration arcing. For example, in plasma chambersfor fabricating flat panel displays on glass substrates, arcing longerthan 10 microseconds poses a risk of damaging a transistor in thedisplay, while arcing longer than 1 second poses a risk of cracking theglass substrate. It may suffice to record the occurrence of a 10microsecond arc in a log, but an arc of 0.1 second duration may requireimmediately shutting down the operation of the plasma chamber to preventa possibly broken substrate from damaging other chamber components.

FIG. 7 shows a variation of the arcing detection algorithm of FIG. 6which includes additional steps 160-164 for detecting whether any arcingpersists for at least N₂ successive samples, where N₂ is an integergreater than the minimum positive result count N, preferably at leastten times greater. For example, if the sampling interval is 1microsecond, then N₂ should be 10⁵ to signal arcing of duration greaterthan 0.1 second.

Steps 160-164 preferably are performed by the DSP, which stores thevalue of N₂ in its memory. The same count used in Steps 112, 120 and 130of the algorithms of FIGS. 4-6 is compared with N₂ in Step 160. If thecount is greater than or equal to N₂, the DSP signals that a prolongedarc has occurred (Step 162), preferably by sending a signal to the PLCover bus 15. The PLC then can initiate remedial action (Step 164), suchas shutting down the plasma chamber, that is more severe than theremedial action taken in response to arcing of shorter duration (Step152).

9. Analog Threshold Detector Circuit

FIG. 8 shows an analog threshold detector circuit that can besubstituted for the DSP 14 in FIG. 1. The threshold detector circuitemploys a conventional analog comparator circuit or operationalamplifier 301-306 for each sensor signal S_(i) or sensor-derived valueV_(i) that is to be compared with a respective arc detection thresholdL_(i) in the arc detection algorithm. The output of each comparatorcircuit 301-306 is the result R₁-R₆ of the corresponding comparison(Step 108 of the arc detection algorithm of any of FIGS. 3-6).

The threshold detector circuit shown in FIG. 8 has six comparatorcircuits which compare six sensor-derived values to six respective arcdetection thresholds L₁-L₆. However, the illustrated circuit can performcomparisons with any number of sensor-derived values or sensor signals,including only one, and in that case would include a number ofcomparator circuits equal to the total number of sensor-derived valuesand sensor signals to be independently compared to respective arcdetection thresholds, i.e., equal to the number of distinct comparisonresults R_(i) to be produced in Step 108.

The threshold detector circuit is connected to receive each of thesensor signals S_(i) that would be connected to the DSP 14 in the FIG. 1embodiment (Step 104). Three of the sensor-derived values V₁, V₃ and V₅received by respective comparator circuits 301, 303 and 305 in the FIG.8 embodiment are the same as the sensor signals S₁, S₂ and S₃ in thepreviously described embodiments, namely, the forward RF power signal25, the reflected RF power signal 27, and the EMF detector signal 48(Step 104). The additional sensor-derived values V₂, V₄ and V₆ receivedby respective comparator circuits 302, 304 and 306 are the respectiveslopes of those signals. We have found an increase in the magnitude ofthe slope of a sensor signal to be a useful indicator of the occurrenceof arcing.

The slopes are derived by connecting each sensor signal 25, 27, 48 tothe input of a respective slope detector circuit 321, 322, 323 (Step106). The output of each slope detector connects to an input of therespective comparator 302, 304, 306. The slope detectors arecommercially available circuits that include a differentiator circuitfollowed by a low pass filter.

Each comparator circuit 301-306 has a positive input and a negativeinput. One input of each comparator circuit is connected to receive avoltage signal proportional to the desired sensor signal S_(i) orsensor-derived value V_(i). The other input is connected to receive avoltage signal proportional to the corresponding arc detection thresholdL_(i). The latter voltage is produced by a respective thresholdadjustment circuit 311-316.

Each threshold adjustment circuit 311-316 can simply be a fixed voltagesource connected to a potentiometer 60 that is manually adjusted toestablish the desired arc detection threshold L_(i) as shown in FIG. 9.Preferably, the potentiometer can be replaced by a digitally controlledresistor ladder network 60 which functions as a digitally controllablepotentiometer. The digital control inputs of the ladder network 60preferably are connected to the PLC 12 via communications bus 15,thereby allowing the PLC to adjust each threshold (Steps 100 and 102 inFIGS. 3-6).

A sensor signal S_(i) or sensor-derived value V_(i) that normally isless than its corresponding arc detection threshold in the absence ofarcing, and that increases in response to arcing, should be connected tothe positive input of its corresponding comparator circuit. Itscorresponding threshold adjustment circuit should be connected to thenegative input. Examples are the reflected RF power signal 27 (FIG. 2C),its slope V₄, the EMF detector signal 48, and its slope V₆. Conversely,a sensor signal S_(i) or sensor-derived value V_(i) that normally isgreater than its corresponding arc detection threshold in the absence ofarcing, and that decreases in response to arcing, should be connected tothe negative input of its corresponding comparator circuit. Itscorresponding threshold adjustment circuit should be connected to thepositive input. Examples are the forward RF power signal 25 (FIG. 2B)and its slope V₂.

This embodiment, in which the analog threshold detector circuit of FIG.8 is substituted for the DSP 14 in FIG. 1, can be used to perform any ofthe variations described above of the arc detection algorithms shown inFIGS. 3-6. The analog threshold detector circuit of FIG. 8 performsSteps 104, 106 and 108 of the arc detection algorithm as describedabove, and the PLC 12 is programmed to perform the other steps of thealgorithm. Because the analog threshold detector circuit performs thecomparisons continuously instead of at sampling intervals, Steps 102-140which are shown in FIGS. 3-6 as being performed repeatedly in a flowchart loop can be performed continuously with the analog thresholddetector circuit. If the comparison results R_(i) produced by the analogthreshold detector circuit are connected to interrupt inputs of the PLC,the PLC can respond immediately to a positive result R_(i) (Steps120-152) rather than responding only after a sampling or pollinginterval has elapsed.

10. Suppression of Arcing

In addition to detecting arcs as described above, the ADSC 10 preferablyis programmed to implement an algorithm for suppressing arcs, by whichwe mean minimizing the duration of an arc after the ADSC has detectedthe arc. In brief, our method of arc suppression is to turn off the RFpower provided by the RF power source 20 to the plasma chamber for ashort time, which usually quenches (stops) the arcing, and then returnthe RF power to its previous level.

The circuit shown in FIG. 1 is capable of implementing our arcsuppression method. The RF power source 20 includes an “enable” input 23which receives a binary signal whose on and off states determine whetherthe output of the RF power source is on or off. When the enable inputsignal 23 is in the “on” state, the RF power source outputs a powerlevel designated by the RF power set point input 21 as described above.When the enable input signal 23 is in the “off” state, the RF powersource provides zero power at its output 22 regardless of the RF powerset point signal 21.

The ADSC 10, preferably the DSP 14, produces a binary RF power enablesignal 70 that is connected to the enable input 23 of the RF powersource. Normally, the DSP maintains the RF power enable signal in the“on” state so that the RF power source outputs whatever level of RFpower is designated by the RF power set point signal 36 from the processcontroller 34.

The arc suppression algorithm performed by the circuit of FIG. 1 isshown in FIG. 10.

The arc suppression algorithm is not performed unless the previouslydescribed arc detection algorithm (Steps 100-150 of FIGS. 3-6) detectsarcing (Step 150).

When the arc detection algorithm performed by the DSP determines thatarcing is occurring (Steps 150-152), the DSP begins performing the arcsuppression algorithm by clearing (resetting to zero) a count “C” storedin the memory of the DSP that represents the number of attempts toquench the arc (Step 200). At this time the DSP also changes the RFpower enable signal 70 to the “off” state, which commands the RF powersource 20 to reduce its power output 22 to zero (Step 201). Steps 200and 201 can be performed simultaneously or in either order.

The DSP maintains the RF power enable signal 70 in the “off” state for apredetermined time period referred to as the arc suppression time periodT_(S) (Step 201). The duration of the time period T_(S) preferably islong enough to achieve a non-negligible likelihood that the arcing willremain quenched—i.e., the arcing will not resume—when the original RFpower is restored after this time period T_(S).

In the preferred embodiment, the default value of the arc suppressiontime period T_(S) is 2 milliseconds. More generally, the time periodT_(S) preferably is less than or equal to 20 ms, and more preferablyless than or equal to 5 ms.

After the expiration of the reduced power time period T_(S), the DSPchanges the RF power enable signal 70 to the “on” state (Step 202), inresponse to which the output 22 of the RF power source 20 returns to thepower level designated by the RF power set point signal 36 from theprocess controller 34, i.e., the RF output power returns to its normallevel for the current step of the process being performed in thechamber.

After the RF power is restored, the DSP tests whether the arcing hasbeen quenched (eliminated) by performing Steps 104-108 of the arcdetection algorithm of FIGS. 3-6 (Step 203). If all of the comparisonsin Step 108 have a negative result, the arc suppression is considered tohave succeeded (“All Negative” outcome of Step 203 in FIG. 10).Accordingly, the DSP clears any arcing alert signal that may have beenset in Step 150 (Step 210), and discontinues performing the arcsuppression algorithm of FIG. 10 and resumes performing the arcdetection algorithm of FIGS. 3-6, beginning at Step 100.

Conversely, if any of the comparisons in Step 108 have a positiveresult, the arc suppression is considered unsuccessful (“One or MorePositive” outcome of Step 203), either because the arcing never stoppedor because the arcing recurred when the RF power returned to its normallevel. In either case, the DSP increments by one the aforesaid count “C”stored in its memory (Step 204), so that the count indicates the numberof times Step 201 was performed in an unsuccessful attempt to suppressthe present arcing.

Next, the DSP compares the count “C” to a predetermined maximumpermitted number Ns that also is stored in the memory of the DSP (Step205). If the number of unsuccessful attempts to suppress the arc is lessthan or equal to the maximum permitted number N_(S) (“No” outcome ofStep 205), then the DSP repeats the arc suppression algorithm beginningwith Step 201, bypassing the step of clearing the count in Step 200.

Conversely, if the number of unsuccessful attempts to suppress the arcis greater than the maximum permitted number N_(S) (“Yes” outcome ofStep 205), the DSP sends an arc alert signal to the PLC viacommunications bus 15 (Step 206), in response to which the PLC can beprogrammed to take remedial action. Examples of such remedial actionsare the remedial actions (2)-(4) described near the end of the precedingSection 2 entitled “Detection of Arcing”.

A possible variation of the described algorithm would be to employ acriterion other than a fixed number of attempts N_(S) to determine whento stop repeating the arc suppression algorithm in Step 205, or even toomit any such criterion (Steps 204 and 205) and simply continue the arcsuppression algorithm indefinitely until the arcing is eliminated.

If the RF power source does not have an “enable” input 23, the arcsuppression algorithm can be implemented by interposing an AND gate,switch, or multiplexor (MUX) 40 between the RF power set point output 36of the process controller 34 and the power set point input 21 of the RFpower source 20.

FIG. 11 shows an embodiment using a multiplexor 40 having two datainputs. The first data input is connected to receive the RF power setpoint signal 36 produced by the process controller 34; its valuerepresents the RF power that the process controller determines isdesired at the current step in the process being performed in the plasmachamber 30. The second data input of the multiplexor is connected to afixed electrical voltage representing a set point power of zero (shownas a zero character “0” in FIG. 11).

The AND gate, switch, or multiplexor 40 has a control input connected toreceive the “RF power enable” binary signal 70 from the DSP 14. When theRF power enable signal is in its “on” state, the AND gate, switch, ormultiplexor 40 connects the RF power set point signal 36 to the powerset point input 21 of the RF power source 20. When the RF power enablesignal is in its “off” state, the AND gate, switch, or multiplexor 40couples the aforesaid zero value to the power set point input 21.

FIG. 2E shows the power set point input 21 alternating between itsnormal value and zero during the time interval t₁-t₂ when the DSP isperforming the arc suppression algorithm. FIG. 2E also shows the powerset point input 21 at a high level during time intervals t₀-t₁ and t₂-t₃during the absence of arcing, and at a lower level dictated by theprocess controller 34 during time interval t₃-t₄.

A possible variation of the design shown in FIG. 11 is to suppress thearcing by reducing the RF power to a level greater than zero butsubstantially lower than the set point power specified by the set pointsignal 36 provided by the process controller 34. This can be implementedby replacing the zero signal “0” connected to the second input of themultiplexor 40 with a voltage source whose value represents such reducedpower level.

FIG. 12 shows a further variation in which the DSP is programmed togenerate an RF power set point signal 72 that is connected to the setpoint input 21 of the RF power source 20, eliminating the need for themultiplexor 40 and the connection between the process controller 34 andthe RF power source 20. In the embodiment of FIG. 12, the DSP isprogrammed to set the power set point signal 72 to the same value as thepower set point signal 36 received by the DSP from the processcontroller during times when the DSP would have set the RF power enablesignal 23 to its “enable” value in the embodiments of FIGS. 1 and 11.Conversely, during times when the DSP would have set the RF power enablesignal 23 to its “disable” value in the embodiments of FIGS. 1 and 11,the DSP in the embodiment of FIG. 12 is programmed to set the power setpoint signal 72 either to zero or to a reduced level to suppress arcingas described in the preceding paragraph.

1-23. (canceled)
 24. An electrical circuit for detecting an abnormalchange in an electrical load to which an RF power source supplies power,comprising: an RF power source having an RF output to which it suppliesRF power, the RF output being adapted for connection to an electricalload; a reflected RF power sensor electrically coupled to the RF outputof the RF power source, wherein the reflected RF power sensor produces areflected power sensor signal responsive to the amount of reflected RFpower at the RF output of the RF power source; and a threshold detectorcircuit connected to receive the reflected power sensor signal from thereflected RF power sensor, wherein the threshold detector circuitdetects when the rate of change of the reflected power sensor signalexceeds a threshold.
 25. An electrical circuit according to claim 24,further comprising: a circuit for reducing the power being supplied bythe RF power source to the RF output in response to the thresholddetector detecting that the rate of change of the reflected power sensorsignal has exceeded said threshold.
 26. An electrical circuit accordingto claim 24, further comprising: a circuit for discontinuing the supplyof RF power by the RF power source to the RF output in response to thethreshold detector detecting that the rate of change of the reflectedpower sensor signal has exceeded said threshold.
 27. An electricalcircuit according to claim 24, wherein: the RF power source furtherincludes a control input, wherein the RF power source supplies to the RFoutput a level of RF power controlled by a signal received at thecontrol input; and the threshold detector circuit includes an outputconnected to the control input of the RF power source, wherein, inresponse to the threshold detector detecting that the rate of change ofthe reflected power sensor signal has exceeded said threshold, thethreshold detector circuit produces a signal at the control input of theRF power source that controls the RF power source to reduce said levelof RF power.
 28. An electrical circuit according to claim 24, wherein:the RF power source further includes an enable input, wherein the RFpower source turns on and off its supply of RF power to the RF output inresponse to a signal received at the enable input; and the thresholddetector circuit includes an output connected to the enable input of theRF power source, wherein, in response to the threshold detectordetecting that the rate of change of the reflected power sensor signalhas exceeded said threshold, the threshold detector circuit produces asignal at the enable input of the RF power source that controls the RFpower source to turn off said supply of RF power to the RF output. 29.An electrical circuit according to claim 24, further comprising: aplasma chamber; wherein the RF output is coupled to the plasma chamber.30. An electrical circuit according to claim 24, wherein the thresholddetector circuit comprises a computer processor that is programmed tocompare the rate of change of the reflected power sensor signal withsaid threshold.
 31. An electrical circuit for reducing an RF powersupplied to a plasma chamber in response to an abnormal change withinthe plasma chamber, comprising: an RF power source having an RF outputconnected to supply power to the interior of the plasma chamber; areflected RF power sensor electrically coupled to the RF output of theRF power source, wherein the reflected RF power sensor produces areflected power sensor signal responsive to the amount of reflected RFpower at the RF output of the RF power source; a threshold detectorcircuit connected to receive the reflected power sensor signal from thereflected RF power sensor, wherein the threshold detector circuitdetects when the reflected power sensor signal exceeds a threshold; anda circuit for reducing said power being supplied by the RF power sourceto the plasma chamber in response to the threshold detector detectingthat the reflected power sensor signal has exceeded said threshold. 32.An electrical circuit according to claim 31, wherein: the RF powersource further includes a control input, wherein the RF power sourcesupplies to the plasma chamber a level of RF power controlled by asignal received at the control input; and said circuit for reducing saidpower includes an electrical connection between the threshold detectorcircuit and the control input such that, in response to the thresholddetector detecting that the rate of change of the reflected power sensorsignal has exceeded said threshold, the threshold detector circuitproduces a signal at the control input of the RF power source thatcontrols the RF power source to reduce said level of RF power.
 33. Anelectrical circuit according to claim 31, wherein: the RF power sourcefurther includes an enable input, wherein the RF power source turns onand off its supply of RF power to the plasma chamber in response to asignal received at the enable input; and said circuit for reducing saidpower includes an electrical connection between the threshold detectorcircuit and the control input such that, in response to the thresholddetector detecting that the rate of change of the reflected power sensorsignal has exceeded said threshold, the threshold detector circuitproduces a signal at the enable input of the RF power source thatcontrols the RF power source to turn off said supply of RF power to theplasma chamber.
 34. An electrical circuit according to claim 31, whereinthe threshold detector circuit comprises a computer processor that isprogrammed to compare the reflected power sensor signal with saidthreshold.
 35. An electrical circuit for detecting an abnormal change inan electrical load to which a power source supplies electrical power,comprising: a power source having an output to which it supplieselectrical power, the output being adapted for connection to anelectrical load; a sensor coupled to the output of the power source orthe electrical load so as to produce a sensor signal responsive to saidelectrical power or to an electromagnetic condition of the electricalload; a threshold detector circuit connected to receive the sensorsignal from the sensor, wherein the threshold detector circuit detectswhen a value derived from the sensor signal crosses a threshold; andwherein the sensor comprises: (i) a magnetic sensor coupled to theelectrical load, (ii) an RF distortion sensor coupled to the output ofthe power source, (iii) a harmonic sensor coupled to the output of thepower source, or (iv) an electrical control signal coupled to anelectrically tunable RF matching network connected between the output ofthe power source and the electrical load.
 36. An electrical circuitaccording to claim 35, wherein: the electrical load is a plasma chamber.37. An electrical circuit according to claim 35, wherein: the electricalload is a plasma chamber; and the sensor comprises a magnetic sensorcoupled to the plasma chamber so as to be responsive to a magnetic fieldwithin the plasma chamber.
 38. An electrical circuit according to claim35, further comprising: a circuit for reducing said power being suppliedby the power source in response to the threshold detector detecting thatsaid value derived from the sensor signal has crossed said threshold.39. An electrical circuit according to claim 35, wherein: said valuederived from the sensor signal is the sensor signal.
 40. A method ofdetecting an abnormal change in an electrical load to which a powersource supplies electrical power, comprising the steps of: supplyingelectrical power from a power source to an electrical load; producing asensor signal responsive to said electrical power or to anelectromagnetic condition of the electrical load; and detecting when avalue derived from the sensor signal crosses a threshold; wherein thestep of producing the sensor signal comprises producing said sensorsignal responsive to: (i) a magnetic field of the electrical load, (ii)an RF distortion of the output of the power source, (iii) a harmoniccomponent of the output of the power source, or (iv) an electricalcontrol signal coupled to an electrically tunable RF matching networkconnected between the output of the power source and the electricalload.
 41. The method of claim 40, wherein: the electrical load is aplasma chamber.
 42. The method of claim 40, wherein: the electrical loadis a plasma chamber; and the step of producing the sensor signalcomprises producing said sensor signal responsive to a magnetic fieldwithin the plasma chamber.
 43. The method of claim 40, furthercomprising the step of: reducing said power being supplied by the powersource in response to the threshold detector detecting that said valuederived from the sensor signal has crossed said threshold.